1. Field of the Invention
This invention relates to thermoplastics and more particularly relates to high hard segment polyurethanes and medical articles therefrom.
2. Background of the Invention
Polyurethanes possess an outstanding balance of physical and mechanical properties and superior blood compatibility compared to other polymers such as silicone rubber, polyethylene, polyvinyl chloride and perfluorinated polymers. As a result, they have come to the fore as the preferred polymeric biomaterials for fabrication of various medical device components. Some important device applications for polyurethanes include peripheral and central venous catheters, coatings for heart pacemaker leads and the Jarvik heart.
As known in the art, polyurethanes are synthesized from three basic components, a diisocyanate, a polyglycol and an extender, usually a low molecular weight diol, diamine or water. If the extender is a diol, the polyurethane consists entirely of urethane linkages. If the extender is water or diamine, both urethane and urea linkages are present and the polyurethane is termed a polyurethaneurea.
The diisocyanate may be aromatic, aliphatic or cycloaliphatic. Commonly used aromatic diisocyanates are toluene diisocyanate and 4,4'-diphenylmethane diisocyanate (MDI). The polyglycol is usually a polyether or polyester having terminal hydroxyl groups for reaction with the diisocyanate.
Polyurethanes develop microdomains conventionally termed hard segments and soft segments, and as a result are often referred to as segmented polyurethanes. The hard segments form by localization of the portions of the polymer molecules which include the isocyanate and extender components and are generally of high crystallinity. The soft segments form from the polyglycol portions of the polymer chains and generally are either noncrystalline or of low crystallinity. One of the factors which determines the properties of the copolymer is the ratio of hard and soft segments. In general, the hard segment contributes to hardness, tensile strength, impact resistance, stiffness and modulus while the soft segment contributes to water absorption, elongation and elasticity.
Polyurethanes chain extended with diols have been extensively studied for biomedical application. Exemplary of important diol extended polyurethanes are: VIALON.TM. (Becton, Dickinson and Co.) PELLETHANE.TM. (Upjohn Chemical Co.,) and TECOFLEX.TM. (Thermedics Inc.). These proprietary products typically have good blood compatibility, but, with the exception of VIALON.TM., generally require processing additives such as antioxidants and detackifiers, a potential disadvantage for use in biomedical articles. They are, however, thermoplastic and therefore may be melt extruded and injection molded.
The TECOFLEX.TM. polyrurethanes are synthesized from nonaromatic diisocyanates and polyglycols and are disclosed in U.S. Pat. No. 4,523,005 to Syzcher. Other U.S. patents disclosing polyurethanes synthesized from polyglycols and-nonaromatic isocyanates are U.S. Pat. No. 4,442,281 to Hentschel et. al. and U.S. Pat. No. 4,917,850 to Gray.
Polyurethanes based on aromatic isocyanates have many salubrious properties which have made them useful for fabrication of medical devices. An MDI-based polyurethane of almost 100% hard segment is claimed to be a tough engineering grade high impact resin. This product is named ISOPLAST.TM. (Dow Chemical Co., Midland, Mich.) and is described by Ehrlich et. al. in Journal of Elastomers and Plastics, 136 (1984) and by Bonk et al. in U.S. Pat. No. 4,822,827.
The present invention is directed to engineering grade thermoplastic polyurethanes synthesized without heavy metal leachable and potentially toxic catalysts and thus are eminently suitable for fabrication of medical devices.